By the end of the book, the pigs have far more in common with humans than with their fellow animals. They have violated all the rules of animalism. They indulge in the following human type behaviors: they live in houses, sleep in beds, wear clothes, drink alcohol, stand on two legs, use money, and carry whips. Like the humans, they work their farm animals very hard and keep them in primitive conditions. Therefore, having so much in common, it makes sense that the pigs would want to invite the humans over and spend time with them.

The pigs play cards with the humans and laugh when the humans compare the pigs' animals to the human's lower classes. The pigs state they are the owners of the farm, and have decided to change its name to the Manor Farm. In fact, by the end of the tale the pigs have become indistinguishable from the humans, which the animals realize as they look in the window of the farmhouse:

The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.

In chapter 10 of Animal Farm, the pigs invite the humans to inspect Animal Farm and further open diplomatic relations between the pigs and the humans. The pigs are tired of being thought of as different from human farms, so they go to great lengths to impress the farmers. The pigs begin wearing human clothes and even learn to walk on their hind legs. They also rename the farm "Manor Farm." Their efforts pay off. The farmers praise the pigs' accomplishments and note that the "lower animals" are much like the "lower classes" among the humans. Mr. Pilkington and Napoleon share a toast, and the pigs and farmers sit down to a game of cards. By the end of the chapter, the other animals are unable to distinguish the pigs from the humans.

Why are humans brought back to the farm in Animal Farm?

Some background information is important to answer this question. 

Running a farm proves to be more difficult than the animal think. For one thing, their supplies begin to run low. For this reason, Napoleon, to the surprise to all to all the other animals, announces that he will work with humans. Mr. Whymper will help him obtain the supplies that the needs to run the farm. From this point, the animals start trading with humans. 

When the animals are confused, the silver-tougued, Squealer, reminds the animals that there was no rule in animalism that prohibited the working with humans. In fact, he goes one step forward by saying that if they think that this new development is wrong, then they have been influenced by the traitor, Snowball. 

In light of this point, it is clear that humans are back for two reasons. First, the animals need humans to have a successful farm. Second and more importantly, humans prove to be good trade partners. As the animals begin to produce, the pigs can now sell them to humans. All of this proves to be profitable, and in the end profits matter most for Napoleon and the other pigs.